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Union of common iliac veins

The union of the common iliac veins represents the anatomical point where the right and left common iliac veins merge to form the inferior vena cava (IVC). This confluence is located at the level of the fifth lumbar vertebra (L5), just to the right of the midline and posterior to the right common iliac artery.

The common iliac veins are formed by the union of the external iliac vein (draining the lower limb) and the internal iliac vein (draining the pelvis). Their union ensures the major venous return from the pelvis, abdominal wall, and lower extremities toward the heart.

This confluence is clinically significant as a common site of venous compression (May–Thurner syndrome), thrombosis, extension of pelvic malignancy, and trauma-related venous injury. It also serves as a critical anatomical landmark in interventional radiology and vascular surgery.

Synonyms

  • Iliocaval junction

  • Iliac vein confluence

  • Origin of the inferior vena cava

Function

  • Collects venous blood from both lower limbs, pelvis, and lower abdominal wall

  • Drains into the inferior vena cava, which carries blood to the right atrium

  • Functions as a venous crossroads between systemic and pelvic circulations

  • Provides anatomical landmark in surgical and endovascular procedures

MRI Appearance

T1-weighted images:

  • Veins appear as tubular hypointense structures surrounded by retroperitoneal fat

  • The IVC origin is well visualized at L5

T2-weighted images:

  • Flowing blood: signal void

  • Thrombosis or slow flow: intraluminal hyperintense signal

  • Surrounding fat and lymphatic structures provide contrast

STIR:

  • Suppresses fat, making veins more conspicuous

  • Venous wall inflammation, edema, or perivascular pathology appear hyperintense

T1 Fat-Suppressed Post-Gadolinium:

  • Normal confluence enhances brightly and homogeneously

  • Thrombosis: filling defect with absent enhancement

  • Useful for detecting tumor invasion, inflammatory thrombophlebitis, or collateralization

MRV (Magnetic Resonance Venography):

  • Normal signal: flowing venous blood appears bright hyperintense

  • Thrombosis: hypointense, non-enhancing filling defect

  • Clearly delineates the confluence of common iliac veins into IVC

  • Useful for diagnosing May–Thurner syndrome, DVT, and venous obstruction

CT Appearance

Non-contrast CT:

  • Confluence appears as tubular soft-tissue density anterior to L5

  • Can detect calcification, mass effect, or large thrombosis

CT Post-Contrast:

  • Normal veins enhance with contrast

  • Thrombosis: intraluminal filling defect

  • Masses encasing the confluence (retroperitoneal tumors, lymphadenopathy) visible

CTV (CT Venography):

  • Normal signal: lumen opacifies as bright hyperdense structure at the iliocaval junction

  • Thrombosis: non-opacified intraluminal filling defect with or without collateral veins

  • Excellent for mapping iliac compression, May–Thurner syndrome, venous anomalies, and surgical

CT image

Union of common iliac veins CT axial

CT image

Union of common iliac veins CT coronal

MRI image

Union of common iliac veins MRI axial anatomy image-img-00000-00000